Don't count on getting the specific model of car you want. What you will usually get is a car from the "class", or price band, of the one you have booked.
I'm not sure that Just Rentals will charge for out of hours collection. Only the big international companies (Hertz, etc.) have desks inside the terminal. All the other, cheaper, companies meet you at the exit and walk you across the road to their compounds in the car park opposite. They keep an eye on flight arrivals and only come out when yours has landed, and you have had a chance to clear passport control and collect your luggage.
if you return the car out of hours, you park it and leave the key at the car park entrance.